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[email protected] joe@mich.com is offline
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Default overvoltage on audio circuits

On Mon, 4 Nov 2019 14:44:22 -0800 (PST), Phil Allison wrote:

wrote:

---------------------

**This raving nutter needs dealing with.

Since you are an audio expert,


** I am, and you are very clearly not.


OK,, let's look at one of the few pearls of wisdom where you attempted to actually provide supporting calculations for one of your statements

YOU
With 3.3nV and 1pA per rtHz of input noise, the best impedance is 3.3kohms.


ME
How are you coming up with these numbers?
Did you get them by dividing the input voltage noise density by the input current noise density? If so, what do you think that gives you?


YOU
** It gives you a good guide to the source resistance that gives the best NF.
Is that news to you ?


ME
Yes it is. I've never known anyone to do that before, let alone think it means anything. I could put a Darlington on the inputs, get the shot noise
down 0.005na/rtHz. Would you then come up with 3.3Mohms as meaning something?


YOU
** Proves you have no real understanding of input noise.

Just a plie of rote learned irrelevances.


Please go into this in greater detail. My understanding is that when using amplifiers, especially audio, the noise figure is a function of the
application - impedance levels, feeback and other external circuitry, closed loop gain setting. etc. Therefore NF is usually not specified on the data
sheet and instead the equivalent voltage and current densities at the input are specified, which allows the user to determine the added noise of the
amp or gain block Since the voltage noise density and current noise density are caused by different mechanisms, they are independent and are treated
separately and their results added by superposition. So, as shown before,

For the microphone preamp app previously given, using a typical microphone impedance of 150 ohms, and choosing a gain of 1000, then

mic noise amp input voltage noise amp input current noise
density density density

SSM (1.6 nV/rtHz) (1 nV/rtHz) (2pA/rtHz× 150 ohms)

OPA (1.6 nV/rtHz) (2.7 nV/rtHz) (1pA/rtHz x {150+10/(10+1000)}

total input noise density

SSM = sqrt{(1.6 nV/rtHz)^2 + (1 nV/rtHz)^2 + (2pA/rtHz× 150 ohms)^2} = 1.96nV/rtHz

OPA = sqrt{(1.6 nV/rtHz)^2 + (3.0 nV/rtHz)^2 + (1pA/rtHz x [150+10/(10+1000)]^2} = 3.0nv/rtHz

and over the audio band the total input referred noise is SSM = 0.28nV OPA = 0.43nV

I think these numbers give the user everything he needs to determine the noise levels and noise added by the amp. However, you say the user needs to
consider NF and you say 3.3Kohms is the best source resistor for best NF for the OPA and you arrived at that number by dividing the input voltage
noise density by the input current density. So,

Now I have a few questions. 1) please explain the physical significance of dividing the input noise density by the input current density.
2) please explain how the 3.3Kohm source resistor you arrive at by doing this improves the noise or any
other performance parameter. please show calculations showing lower numbers than those above
3) Since the SSM has a higher input noise current density and would therefore give a lower resistor for
this calculation, does this mean better amps have higher input noise current densities?
4) As mentioned earlier, if I use a Darlington follower on the OPA inputs and lower the input current
density to 5pa, would you now use a 3.3Mohm source resistor?

Since you are an expert and I am not, so you say, perhaps you can explain Ohm's Law to me. See, I always thought Ohm's Law stated that the voltage
across an impedance (resistance) was linearly proportional to the current through it, and the constant of proportionality was the value of the
resistance. Since the input noise voltage density is due thermal noise of the parasitic base resistance,and the input current noise density is due to
the shot noise of the collector divided by transistor beta, and the component due to base shot noise times base resistance is negligible in this case,
then the input noise voltage and current are independent and uncorrelated. I don't think Ohm's Law defines a resistor in the case of independent
values for voltage and current. I always thought they had to be linearly related to define a resistance.

Please try to answer in a mature way and support your position with technical calculations or references. Try to provide a technical discussion. If
you have some knowledge new to me, I'd like to learn it. See if you can avoid deleting large parts of posts you find inconvenient and resorting to
insults and name calling and stating over and over that you are an audio expert and I'm not.

However, I don't really expect to get a rational technical answer to the questions I've asked. I hope I'm surprised, but based on your past responses
in this and other threads, I'm getting a picture of how you click. Look at this below

How sickening.


I said you had an attitude and often
called past posters idiots when they were looking for help.


** And you're real sure they were not all idiots ?

How would YOU know that?

I doubt you have any way to tell.


Is this the way you live your life? Did you not get enough love when you were little?